In one type of laser printing system, a photosensitive media is placed on a drum and is written to by a laser diode. A light beam from the laser is typically deflected from a polygon or galvanometer, and focused through an imaging lens. As the image is written pixel by pixel, small areas of a photosensitive material are exposed.
The amount of laser energy delivered is important and variations in power level can introduce artifacts in the image printed on the media. For example, using three colored lasers to write to film, variations in power of one of the lasers can introduce an artifact called "banding." In some applications, laser optical power must be controlled to better than 0.5% accuracy in order to obtain a reasonable image quality.
Optical power is affected by many parameters, such as laser diode driving current and operating temperature. In order to ensure that a laser operates at a stable condition, an operating temperature is chosen in which the laser operates at a wavelength which is relatively constant despite minor temperature variations. For example, FIG. 1 shows that a laser operating in the temperature range of 19.degree.-22.degree. C. at a wavelength of approximately 685 nm would be relatively stable. Outside this range there would be variations in intensity of the laser output power and variations in the polarization of the laser output power indicated by the erratic wavelength.
Another problem which may cause variations in laser power output is caused by optical feedback, which is unwanted light reflected back into a laser by optical elements external to the laser. Optical feedback can disturb laser operation and cause intensity fluctuations which may amount to as much as 10% or 20%. As more components are added, such as in an imaging lens and rotating polygon, the stable range in which the laser can operate can be decreased significantly.
Other factors may affect the stability of laser operating systems. For example, characteristics of some components change with age, and small contaminates may accumulate on the surfaces of the optical lens. This change can cause variations in reflectivity which results in optical feedback to the laser.
Past attempts to stabilize laser performance have met with mixed results. For example, thermoelectric coolers have been used to prevent ambient temperature drift. However, lasers still may change modes because laser characteristic changes or elements shift, causing optical feedback. Also thermo-electric coolers add additional cost and complexity.
Another method of stabilizing lasers is using back facet photosensors which detect a portion of the light leaving a back facet of the laser to control laser output. This has not been entirely successful because the layers of dielectric mirror coating on the back facet of the laser are wavelength specific. Therefore, small changes in the wavelength of the light leaving the back facet can result in large changes in power to the back facet sensor.
Another attempt at stabilization of lasers has used radio frequencies (RF) to stabilize lasers at low power, for example, in the range of 1 to 2 mW. These low power RF stabilization schemes, however, are not suitable for high power laser stabilization because of intensity control problems. This type of RF stabilization in a high power laser has a possibility of back-biasing the laser diode and destroying it. See U.S. Pat. Nos. 5,197,059; 5,386,409; and 5,495,464. Other undesirable features in RF control are decreased lifetime and overdriving of the laser. See U.S. Pat. Nos. 5,495,464 and 5,175,722.
It is, therefore, desirable to stabilize a laser against changes in temperature, current variations, effects of aging, and optical feedback.